Although most optical fibers consist of doped or undoped glass core surrounded by a doped or undoped glass cladding, there are several cases in which it may be advantageous to use a polymeric material as cladding instead of siliceous glass. Polymeric materials are flexible, so they do not break as easily when the fiber is bent. Besides, the refractive index of polymers can be made lower than that achievable by doping silica with fluorine or boron. This lower refractive index provides a larger numerical aperture, thereby increasing the acceptance angle for the incoming radiation. U.S. Pat. No. 4,511,209 and European Pat. No. 0 333 464 B1 disclose polymer compositions suitable for use as claddings for optical fibers.
Other devices that require low refractive index coatings are optical fiber lasers and amplifiers (cladding pumped lasers) such as those described in co-assigned U.S. patent application Ser. No. 08/561,682 and in U.S. Pat. No. 4,815,079. These devices comprise a doped glass core, a doped or undoped glass cladding and an outer polymer cladding. The core is pumped by a laser guided through the inner (glass) cladding; the outer (polymer) cladding has an even lower refractive index so that the combination inner/outer cladding can act as a waveguide. The numerical aperture and, therefore, the output power of this device is a function of the difference in the refractive indices between the inner and outer claddings. In this sense, it is desirable to have a polymer cladding with the lowest possible refractive index.
The process currently used to coat glass fibers with polymers comprises applying a UV-curable formulation with a die. Acrylate-functional formulations are the most widely used coating materials because of their extremely fast curing rates, ease of synthesis and commercial availability. Commercially available low-refractive index polymer coating formulations have indices on the order of 1.38-1.42 (for example "OPTI-&gt;CLAD" supplied by Optical Polymer Research, Inc., Gainesville, Fla.). The patent literature shows coatings with refractive indices in the range 1.37-1.43 (U.S. Pat. No. 4,508,916, U.S. Pat. No. 4,511,209, U.S. Pat. No. 4,971,424, U.S. Pat. No. 5,024,507, and U.S. Pat. No. 5,239,026). There are some newer materials that have much lower refractive indices (1.31-1.33). Nevertheless, there is no practical process currently available to coat these materials onto a glass fiber because their viscosities are too low for a standard die-coating application. Additionally, these materials are highly fluorinated and commercially available photoinitiators (required for a UV-curing formulation) are typically not soluble in the lowest refractive index materials.
One means for circumventing these problems is disclosed in European published patent application 0 521 360 A2. However, the approach of the reference is quite complicated.
B-staging is a procedure commonly used with epoxy resins to advance the reaction to limited extent. For instance, in laminated composites, a liquid epoxy resin is first "B-staged" to a tacky sheet that can then be stacked with other sheets and further cured. However, it is not believed that such B-staging has been utilized in the case of acrylates or methacrylates in order to intentionally raise their viscosity significantly.
It is an object of the present invention to provide a new group of materials which are prereacted and which maintain the (meth)acrylate functionality so that they be cured to a final product.
It is a further object of the present invention to provide new (meth)acrylate-containing materials having an increased viscosity or which are materials ranging from soft gels to solids wherein such materials can be cured to a hard product.
It is a still further object to provide a cladding-pumped laser having an improved outer polymer cladding.
Yet another object of the present invention is to provide ultra-low refractive index photo-curable coatings.
It is a further object to provide plastic-clad optical fibers, wherein the cladding material is the free-radical polymerization product of the aforementioned coating material.